The synthetic zeolite ZSM-5 has been described by Argauer et al in U.S. Pat. No. 3,702,886 and by many others since 1972. ZSM-5 is a zeolite which crystallizes in the orthorhombic system with unit cell dimensions of about:
a.sub.0 =20.1 Angstrom units
b.sub.0 =19.8 Angstrom units, and
c.sub.0 =13.4 Angstrom units
The unit cell contains 96 atoms of silicon and aluminum, each tetrahedrally coordinated with oxygen which are often referred to as silicon and aluminum tetrahedrons (also silica and alumina tetrahedrons). As pointed out in the basic Argauer et al patent, silicon can be replaced with germanium and aluminum can be replaced with gallium to still obtain ZSM-5. As used herein the term silica is to include germania and mixtures of germania and silica and the term alumina is to include gallia and mixtures of gallia and alumina. The proportion of silicon to aluminum may vary depending on composition. For example, a ZSM-5 product with a SiO.sub.2 to Al.sub.2 O.sub.3 mole ratio of 46:1 will contain, on average, 92 silicon and 4 aluminum tetrahedrons per unit cell. These tetrahedrons form a rigid covalent framework. The zeolite may contain other substances or ions which are not part of the framework. For example, it may include cations, water, organic molecules, hydroxyl ions or gases. These substances are present in the pores within the crystal structure. ZSM-5 crystals are traversed by two sets of channels or pores. A set of essentially straight pores which run parallel to the "b" axis and a set of pores which zigzag or undulate in the direction parallel to the "a" axis. The two sets of pores or channels intersect on a regular and repetitive manner such that each unit cell includes four such intersections.
Thus, a typical unit cell can be represented in simple terms by the following symbolic notation: ##STR1## in which
M represents monovalent cations such as alkali metals, organic bases, or H.sup.+.
.quadrature. represents the pore intersections, and the other symbols have their usual chemical meaning.
Note that the summation of silicon and aluminum equals 96 (the number of tetrahedrons per unit cell) and that the number of monovalent cations equals the number of aluminum atoms.
One can further refine the symbolic representation of a typical unit cell by indicating the presence of organic bases such as, for example, normal tetrapropyl ammonium ion [(n-C.sub.3 H.sub.7).sub.4 N].sup.+ hereinafter designated by the letter Q, which, because of its size and shape, is centered in the intersections unable to move within the rigid framework and, therefore, blocking diffusion or flow of other molecular species or ions.
The representation takes the following form: ##STR2## in which
.quadrature. represents the "open" intersections which are either empty or filled with small molecules or ions which are capable of and allow diffusion or flow through the channels and intersections, and
Q represents those intersections which are occupied by large, fixed ions such as tetrapropyl ammonium (represented by Q) which block diffusion and flow through those intersections. Note that the summation of "open" and blocked intersections is four.
Although Formula (2) does not include all of the components of the ZSM-5 zeolite in each and every of its possible forms, it includes all of its key functional components. Formula (2) provides the following important information:
the silica to alumina mole ratio: R=2 (96-x)/x,
the number of blocking templates per unit cell: y,
the number of intersections per unit cell: 4,
the fraction of blocked pore intersections: ##EQU1##
the particular cationic form (Na.sup.+, NH.sub.4.sup.+, H.sup.+, etc.) of the zeolite,
the electrical neutrality of the system, and
a general basis to define stoichiometric proportions in a ZSM-5 product.
Under this notation x indicates the number of aluminum atoms in the unit cell and thus there will be 96-x silicon atoms since the total of these two elements is 96. There are 4 intersections for each unit cell which are represented by the 2 boxes. The one on the far right represents the number of intersections, (y), filled with Q ions, and the other box represents the remaining intersections which do not contain Q ions and they number 4-y. Electrical charge neutrality is maintained since the number of monovalent cations equals the number of aluminum atoms in the structure.
The SiO.sub.2 to Al.sub.2 O.sub.3 mole ratio in ZSM-5 can be varied and ZSM-5 zeolites have been made with very large SiO.sub.2 to Al.sub.2 O.sub.3 mole ratios. Dwyer et al in U.S. Pat. No. 4,441,991 refer to high ratio zeolites disclosed in U.S. Pat. No. Re. 29,948 and equivalents of such zeolites, e.g., silicalite disclosed in U.S. Pat. No. 4,061,724. Dwyer et al indicate the equivalency of these two zeolites is known in the art, as discussed, for example, by Fyfe et al, in Resolving Crystallographically Distinct Tetrahedral Sites in Silicalite and ZSM-5 by Solid State NMR, 296 Nature 530 (Apr. 8, 1982), by Rees in When is a Zeolite Not a Zeolite, 296 Nature 491 (Apr. 8, 1982), and by Bibby et al., in Silicalite-2, a Silica Analogue of the Aluminosilicate Zeolite ZSM-11, 280 Nature 664 (Aug. 23, 1979). As used herein- the term ZSM-5 zeolite also includes silicalite.
In the classical synthesis of ZSM-5, following the teachings of the Plank et al patent (U.S. Pat. No. 3,926,782), tetrapropylammonium ion (0) is used as a template. Crystallization of the silica and alumina tetrahedrons takes place around the Q ions which end up occluded at the intersections of the two sets of pores. The Plank et al synthesis requires the use of large excess of Q ions in relation to the number of intersections in the resulting ZSM-5 structure. As a result, a large proportion of the Q employed ends up in solution in the mother liquor following synthesis and the remainder ends up within the crystal structure.
Symbolically the Plank et al product may be represented in its key features by: ##STR3## in which y is nearly 4, and the degree of blocking is essentially complete.
Essentially all substances or ions of appreciable size, such as for example hydrated sodium ions, will be unable to diffuse freely through the structure until the blocking Q ions are removed. Furthermore, the Q ions cannot be easily removed because their size is large, and their fit is very tight. For all practical purposes the Q ions are occluded and fixed. The only practical way to remove the occluded Q ions from the structure is to break down these large cations through pyrolysis at high temperature and/or oxidation. Once the Q has been removed from the structure, the channels and intersections become open to diffusion of molecules or ions of sizes generally below about 5 Angstroms. For example, hydrated sodium ions may easily diffuse through an aqueous medium and be exchanged by other cations such as, for example, ammonium. Since the most common use of ZSM-5 as a catalyst is in the hydrogen form, its preparation will normally require the following sequence of principal steps:
synthesis using organic materials as the templates,
high temperature calcination to remove organic blocking cations such as Q,
exchange of alkali metal ions with ammonium ion, and
decomposition at elevated temperature of the ammonium ion into the hydrogen form of the zeolite and gaseous ammonia.
Attempts have been made to make ZSM-5 with small crystallite sizes. When used as a catalyst in hydrocarbon conversion these small size crystallites retard catalyst aging during the hydrocarbon processing reactions as disclosed by Plank et al in U.S. Pat. No. 3,926,782. The zeolites in the Plank et al patent were made with either relatively large concentrations of tetrapropyl ammonium bromide (QBr) or a tertiary amine and an alkyl halide (such as bromide) which would form a tetra-alkyl ammonium halide such as QBr when using tripropylamine and propylbromide. The disposal of these materials in the waste reactant mixtures presents environmental pollution problems. Furthermore, the zeolite as formed had to be calcined before being ion exchanged. Haag et al in U.S. Pat. No. 4,326,994 also used large concentrations of a polyalkyl amine and an organic halide (which would combine to form a tetra-alkyl ammonium halide such as QBr) to make small crystallites. Again, it was necessary to calcine the zeolites before they could be exchanged.
In these prior systems using Q ions, the amount of Q employed was very large. For example, in the basic Argauer et al U.S. Pat. No. 3,702,886 the first example uses over 1250% of the amount of Q needed to occupy all intersections on the basis of the unit cell stoichiometry of Formula (2).
ZSM-5 has also been produced from reaction systems containing seeds. Rollmann et al U.S. Pat. No. 4,203,869 disclosed using ZSM-5 crystals as seeds and noted that the successful crystallization required the further presence of Q, the tetrapropylammonium cation. Plank et al in U.S. Pat. No. 4,175,114 used seeds alone or in combination with an alcohol. The alcohols disclosed were aliphatic alcohols and preferably containing 2 to 5 carbon atoms. Illustratively named alcohols were ethanol, propanol, butanol and pentanol. The patentees stated they contemplated that the alcohols could be straight or branched chain. There was no mention of the crystallite size obtained.
ZSM-5 has also been made in a low sodium form so the zeolite need not be ion exchanged prior to use. Rubin et al in U.S. Pat. No. 4,151,189 disclosed using propylamines with stirring to produce a zeolite in the as-synthesized form which had less than 0.14% by weight of alkali metal. There was no discussion of the crystallite size. Plank et al in U.S. Pat. No. 4,341,748 claimed an uncalcined form of ZSM-5 which was capable of substantially complete ion exchange of its original metal cations without prior calcination. The disclosure was a continuation-in-part of the Plank et al U.S. Pat. No. 4,175,114 patent discussed above with regard to seeding and it had the same examples.
ZSM-5 has also been made in reaction systems which do not utilize the large Q ion templates. Taramasso et al in U.S. Pat. No. 4,431,621 disclosed the use of organic substances which contain hydroxyl functions such as alcohols and phenols and more particularly glycols and polyglycols. The patent did not give the crystallite sizes.